Method for removing oxides from alloy powder



United States Patent O flice 3,476,548 Patented Nov. 4, 1969 3,476,548METHOD FOR REMOVING OXIDES FROM ALLOY POWDER Thomas S. Cloran, EastLiverpool, Ohio, assignor to Crucible Inc., a corporation of Delaware NoDrawing. Filed Dec. 9, 1966, Ser. No. 600,379 Int. Cl. B22f 1/00; C23g1/02 US. Cl. 75-.5 6 Claims ABSTRACT OF THE DISCLOSURE It is known toproduce metal-alloy articles, such as highalloy steel articles, byobtaining a powder of the desired alloy; placing the powder, which hasbeen cleaned of contaminating surface oxides, in a protectiveatmosphere, such as a vacuum, and compacting the powder to form thedesired article. Compacting is conventionally performed at elevatedtemperatures and consequently an inert atmosphere is required to preventoxidation of the powder. Oxidation, of course, impairs bonding and moreimportantly results in a final compacted article having poorcleanliness.

For the purpose of cleaning metal powders prior to use in the abovepowder metallurgy operations, it is customary to clean them by removingsurface oxides by operations substantially similar to those that wouldbe used were the alloy in a form such as strip, sheet, or wire. Forexample, the alloy particles are subjected to a treatment in moltencaustic, acid pickling and mechanical scale breaking, such as sonicvibration. These operations may be performed singly or in combinationdepending upon how difiicult the oxide of the particular alloy is toremove. The more difiicult or complex the alloy, the more severe must bethe treatment.

Although the above-described cleaning technique has been found suitablefor many alloy compositions, there are oxides of particular alloyingelements, such as titanium, and aluminum, which are used as alloyingelements in the so-called superalloys that present a particular problem.When the surface of these alloys is made substantially clean and freefrom oxide, upon exposure to an oxidizing atmosphere, such as air,immediate reoxidation will occur. Consequently, when using thesealloying elements in powder metallurgy operations, it is necessary notonly to effect removal of the complex surface oxides prior to compactingthe powders of these alloys into articles, but more importantly it isalso necessary to protect the powders once cleaned from reoxidationprior to compacting.

It is accordingly a primary object of the invention to provide a methodwhereby after cleaning by oxide removal the surfaces of alloy powdersmay be protected against reoxidation in the presence of oxidizingatmospheres prior to compacting.

This and other objects of the invention as well as a completeunderstanding thereof may be obtained from the following description andexamples.

In the practice of the invention, a charge of alloy particles, which areintended for subsequent use in compacting into alloy articles, areplaced in a bath of molten caustic. Any of the well-known caustics, suchas sodium hydroxide and potassium hydroxide, may be used. The moltencaustic is typically at a temperature of about 1000? F., and the powderis maintained therein for about 15 minutes. The particles are removedfrom the caustic and rinsed with water to remove excess caustictherefrom. Thereafter, the particles are transferred to an acid picklingbath, which may be, for example, hydrochloric acid or sulfuric acid.During acid pickling the powder is preferably subjected to sonicvibration to insure complete contact of the particle surface area withthe acid. After pickling the particles are rinsed with water to removeexcess acid. It is to be understood, of course, that more than onepickling operation, followed by water rinsing, may be employed.

After the particles have been pickledl and the excess acid removed bywater rinsing, they are, of course, saturated with water. Upon exposureto the atmosphere in this condition, the surfaces of the powder, whichhave been cleaned of surface oxides by the treatment described above,will begin to reoxidize. Because these oxides, as explained above, aredifficult to remove, adequate removal of oxides, particularly oxides oftitanium and aluminum, thus formed will not result by merely placing thepowder in an evacuated container and heating the same to an elevatedtemperature prior to compacting, which is a customary practice in powdermetallurgy. Consequently, it is essential, if an oxygen-free article isto be produced, to protect the cleaned powders from reoxidation whileexposed to oxidizing atmospheres prior to compacting.

In accordance with the present invention, the cleaned particles, afterbeing subjected to the above-described oxide removal operation, aredried (water removed) and protected from further oxidation in thefollowing manner. The wet particles are placed in a container,preferably that which is to be used during subsequent heating,evacuation, and compacting. The water is then displaced by the additionof an agent, such as methylene chloride. Methylene chyloride, or anydrying agent substituted therefor, such as hexane or toluol, must havethe properties of zero solubility for water, high vapor pressure, and beheavier than water. Upon displacement of the water, which may be removedby decanting, the methylene chloride saturated powder is protectedagainst reoxidation by exposure to an oxidizing atmosphere, such as air.The methylene chloride provides a barrier between the surface of thepowder and the surrounding air. The powder may then be placed in achamber for evacuation and heating to elevated temperature whereupon, inthe conventional manner, pressure may be applied to compact the powderinto the desired alloy article. Since methylene chloride has zerosolubility for water, no oxygen which would otherwise be present fromany water dissolved in the methylene chloride is present during theheating operation. The high vapor pressure of the agent, as indicated byits low boiling point of 40 C., insures complete removal of the agentfrom the powder during evacation and prior to compacting. The vaporpressure of the drying agent must be sufficiently high so that at thetemperatures and pressures prevailing in a particular operation, thedrying agent will be completely removed prior to compacting. If thedrying agent is not completely removed prior to compacting, such willresult in contamination of the final compacted article. It may be seen,therefore, that in accordance with the method of the invention asoutlined above dry, clean, oxide-free powders are protected againstreoxidation prior to compacting to result in a compacted article havingan extremely low oxygen content and a corresponding high degree ofcleanliness.

A charge of a mesh powder of a precipitation hardenable, nickel-chromiumbase alloy having in percent by weight- 12.50 chromium, 4.20 molybdenum,2.20 columbium and tantalum, 6.10 aluminum, 0.80 titanium, 0.12 carbon,0.012 boron, 0.10 zirconium, and balance nickel in a 100 gram quantitywas poured into a sealed end of a A in. pipe about 3 ft. in length.Steel wool was then tapped on top of the charge to secure it at thebottom 2 inches of the pipe. The open end of the pipe was connected to avacuum pump and pumping was begun and continued until the outgassingrate stabilized (about 1 micron per minute). About 12 to 14 in. of theclosed end of the pipe were placed in a furnace at a temperature of 2200F. and maintained therein until the outgassing rate stabilized at about100 microns per minute. The pipe was removed from the furnace and theend containing the charge was compressed by a ZOO-ton press operated atabout 25-ton per square inch. This produced a compact approximately A1in. thick, 2 in. wide, and 3 in. long. Sections of this compact wereobtained and olished specimens were prepared therefrom. The polishedspecimens were examined at magnifications of about 400 Prior to theabove-described treatment, the powder was cleaned by treatment in a hotsodium hydroxide bath (about 850900 F.) for approximately 15 minutes,water rinsed to remove excess caustic, placed in bath of hydrochloricacid (160180 F.) for approximately 5 minutes while being subjected tosonic vibration, and water rinsed.

The powder was divided into a number of 100-gram charges, which were airdried and subjected to the above treatment. Polished specimens of thismaterial exhibited 85-90 percent oxide-free particle boundaries.

The operation was repeated, except that the air-drying step was replacedby an operation wherein the wet powder was placed in a container, andcovered with methylene chloride. The water was displaced and removed bydecanting. The owder saturated with methylene chloride was then treatedas described above. Polished specimens of this compacted materialexhibited 95-100 percent xide-free particle boundaries.

The above experimental work indicates that by drying and protecting thepowder after cleaning and prior to oxidation by using methylenechloride, reoxidation of the powder is prevented to result in a finalcompact having oxide-free particle boundaries, which is not the case ifthe particles are air-dried in the conventional manner.

Although various embodiments of the invention have been describedherein, it is obvious that other adaptations and modifications may bemade by those skilled in the art without departing from the spirit andscope of the appended claims.

What is claimed is:

1. A method for producing a charge of alloy powder free of complexsurface oxides for use in compacting to form an alloy article comprisingtreating said powder with molten caustic, water rinsing said powder toremove excess caustic, acid pickling said powder, water rinsing saidpowder to remove excess acid, removing the water from said powder bydisplacing the same with a dry ng agent, said drying agent being heavierthan water, havmg substantially zero solubility for water and high vaporpressure, whereby said drying agent provides a barrier between thechemically clean surfaces of the powder and the oxidizing atmosphere.

2. A method according to claim 1 wherein said drying agent is methylenechloride.

3. A method according to claim 1 wherein said alloy powder contains anamount of at least one alloying ele- 0 ment selected from a groupconsisting of titanium and aluminum.

4. A method according to claim 1 wherein said drying agent is selectedfrom a group consisting of methylene chloride, hexane, and toluol.

5. A method according to claim 1 wherein said powder is subjected tosonic vibration during acid pickling and displacing of the water withsaid drying agent.

6. A method for producing a charge of alloy powder free of complexsurface oxides for use in compacting to form an alloy article comprisingtreating said powder with molten caustic, rinsing said powder to removeexcess caustic, acid pickling said powder, water rinsing said powder toremove excess acid, removing the water from said powder by displacingthe same with a drying agent, said drying agent being heavier thanwater, having substantially zero solubility for water and high vaporpressure whereby said drying agent provides a barrier between thechemically clean surfaces of the powder and the oxidizing atmosphere,charging said powder saturated with said drying agent to a gas-tightcontainer to remove said drying agent by vaporization thereof, wherebysaid charge of alloy powder is dry and free of complex oxides andprepared for compacting.

References Cited UNITED STATES PATENTS 2,027,532 1/1936 Hardy 75-2112,458,661 1/1949 Webster et al. 1343 2,567,456 9/1951 Webster 134--32,630,393 3/1953 Francis 1343 2,638,424 5/1953 Hansgirg 1343 L. DEWAYNERUTLEDGE, Primary Examiner T. R. FRYE, Assistant Examiner US. Cl. X.R.

